The invention relates to a flux which can be used for dry application, and to the use thereof as a brazing flux.
For many years, it has been known to braze together components of aluminium or aluminium alloys, in particular heat exchangers for the automobile industry, using fluxes based on alkali fluoroaluminate. In such a case, the flux is usually sprayed on to the heat exchangers as an aqueous suspension. In the presence of a brazing composition or of a braze-forming precursor such as silicon powder or potassium fluorosilicate, a stable, non-corrosive compound is formed upon heating the components to a temperature above the melting point of the flux. A process has indeed already been disclosed by published German patent application no. DE-OS 197 49 042 with which the waste water produced in this procedure can be recirculated. However, other process parameters are critical: the concentration of the flux slurry must be monitored, the heat exchangers must be dried before heating, the flux slurries, which are likewise recirculated, may pick up impurities. These disadvantages can be avoided if the flux is applied dry to the components to be joined. This is the case in the dry-flux process. Therein, the dry flux powder is applied to the components electrostatically. The advantage is that no slurries need to be produced, that the concentration of the slurry does not need to be monitored, that it is not necessary to provide a separate drying stage for the components, and that no waste water is produced.